Landfill gas (LFG) is formed due to anaerobic decomposition of organic waste material at landfill sites. LFG mainly consists of CH4 and CO2, both known as greenhouse gases. Cheapest option to avoid emissions is to collect LFG and send it to a flare. However LFG has the potential to serve as a renewable energy resource, as CH4 is a valuable component. To convert LFG to commercial grade CH4, several separation and purification steps are required, e.g. removing the contaminants CO2, H2O, and sometimes H2S.
Several separation technologies are being applied or researched in the field of biogas treatment, such as adsorption, membranes and cryogenics. Every technology has its advantages and drawbacks.
One particular widely applied separation technology is adsorption. Separation of for instance CO2 from gas mixtures by adsorption is based on differences in equilibrium capacities at the adsorbent surface (e.g. zeolite 13X) or on differences in uptake rates (e.g. carbon molecular sieve 3K). An adsorption process normally consists of three packed beds, which are operated in different cycles, e.g. adsorption, evacuation, repressurization and product rinse steps. In Grande and Rodrigues, (Grande C. A., Rodrigues A. E., (2007), Biogas to Fuel by Vacuum Pressure Swing Adsorption I. Behavior of Equilibrium and Kinetic-Based Adsorbents. Ind. Eng. Chem. Res., 46, 4595-4605), a Vacuum Pressure Swing Adsorption process is described with two different adsorbents, i.e. carbon molecular sieve 3K and zeolite 13X to separate CO2 from CH4. A disadvantage of this process is the relatively low purity of the obtained CH4 and the high energy consumption. In addition, in the process of Grande and Rodrigues it is assumed that water and other contaminants have been previously removed, as these compounds may interfere with the sorption of the CO2.
Other processes make use of the triple point of CO2, which is approximately 5.2 bara and −56.7° C., and the fact that liquid CO2 can only exist at certain temperatures and pressures above the CO2 triple point.
In U.S. Pat. No. 7,073,348 is disclosed a process for the capture of CO2 from flue gas at atmospheric pressure by contacting the flue gas with the external surface of a heat exchanger, while evaporating a refrigerant fluid on the inside. Solid CO2 is deposited on the external walls of the heat exchanger. After a certain operating time, the flow of flue gas on the external part of the exchanger and refrigerant fluid on the inside of the exchanger are respectively switched over to a second parallel heat exchanger. The solid CO2 deposited on the externals surface of the first heat exchanger is reheated from −78.5° C. to −56.5° C. at a pressure of 5.2 bar and the CO2 is retrieved as a liquid phase.
Heat exchangers are expensive and have limited area available for heat exchange and deposition of the solid CO2. As the refrigerant continuously provides cold to the evaporator surface, most of the CO2 will deposit on the upstream side of the evaporator, resulting in an inhomogeneous distribution of the solid CO2. Also, due to the build up of the solid CO2 layer the pressure drop over the evaporator is increased significantly during operation. Furthermore, the resistance to heat transfer is increased with the increasing thickness of the deposited solid CO2 layer, resulting in an inefficient use of the refrigerant. Consequently, it is necessary to operate the expensive and relatively sensitive evaporator apparatus at short deposit/removal cycles thereby exposing the evaporator apparatus to rapid changes in temperature, which is disadvantageous from a mechanical point of view.
U.S. Pat. No. 4,265,088 discloses a process for treating hot exhaust gas using two or more packed towers. In the process of U.S. Pat. No. 4,265,088, the hot exhaust gas is introduced in a packed tower, which was cooled to a temperature below the sublimation temperature of CO2. The CO2 is sublimated and thereby captured from the exhaust gas. The sublimated solid CO2 is subsequently removed from the packed tower by applying a vacuum to the packed tower to induce evaporation of the solid CO2. However, such a process can only be used for treating exhaust gases containing low concentrations of CO2. When the exhaust gas contains high concentrations of CO2 the use of a vacuum becomes impractical. Alternatively, U.S. Pat. No. 4,265,088 discloses the use of treated exhaust gas to remove the solid CO2 from the packed tower. However, this has the disadvantage that CO2 is reintroduced in at least part of the treated exhaust gas.
In WO2009047341, a process for separating CO2 from a flue gas is described, which is based on dynamic operation of three parallel packed beds, which are operated in cooling, capture and recovery cycles. In the process of WO2009047341, the flue gas is contacted with a fixed bed of cold particles on which the CO2 sublimates as a solid. Subsequently, the particles with solid CO2 thereon are contacted with a warm gas to heat the particles and evaporate the carbon dioxide. Finally, the heated particles are cooled again.
The process of WO2009047341 is focused on retrieving pure CO2 and therefore the warm gas applied to heat the particles with solid CO2 thereon is itself essentially pure CO2. This results in the need to operate the process in three separate cyclic steps each performed in a separate fixed bed, wherein in the middle step the packed bed CO2 is removed exposing the solid CO2 to temperatures above the sublimation temperature, thereby increasing the temperature of the packed bed, while in the latter step it is required to cool the heated fixed bed back to a temperature below the sublimation temperature of CO2.
There is a need in the art for a process to purify CH4-comprising gases such as landfill gas, which is simpler, less energy intensive and allows for the production of high purity CH4.